Christian Davenport

Hundreds of thousands of pieces of man-made debris are floating around out there, the detritus of more than 50 years of spaceflight. There have been chunks of dead satellites and spent rocket boosters - even a glove that an astronaut dropped in 1965 and a spatula that escaped from a space shuttle in 2006.

Because it zips along faster than a speeding bullet, the trash poses an ever-growing threat to the satellites that help the military communicate and gather intelligence and serve the world's obsession with Google Earth and movies on-demand. Until last year, the Pentagon used what was called a "space fence" to track the junk and warn of potential collisions that make owners scramble to move their satellites out of the way.

George Clooney in the film Gravity, which raised awareness about space junk.

But that system managed to keep up with only a fraction of it all, and it went out of commission last year. Now the US Air Force is poised to take a more modern crack at the problem with a new space fence.

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With a contract expected to be awarded in the next few weeks, the program is designed to be mankind's best effort yet at tracking space pollution. But the new space fence still doesn't provide what many think is the ultimate solution: cleaning up space.

"There's a lot of stuff up there, and the impact of the new space fence will be to track more objects and smaller objects," said Joan Johnson-Freese, a professor of national security affairs at the US Naval War College.

Tracking the debris "is a necessity, but not sufficient," she said. "We need to move on to an active plan for removal."

The fence isn't really a fence, but rather a high-frequency radar like a flashlight beam in a dark room that illuminates the bits of dust swirling around. All those little bits are then catalogued and tracked as they pass through the radar again and again, until analysts, using massive computer databases, can predict where those pieces of debris will be in the future and when they might come close enough to collide with something.

The new system is vital, officials say, because it will be able to track more and much smaller pieces of debris, which can whip around the globe as fast as 27,000 km/h. At that velocity, even something just a centimetre around would pack a punch like a bowling ball traveling at 480 km/h, according to NASA.

"It's absolutely essential," said Todd Harrison, a defence analyst at the Centre for Strategic and Budgetary Assessments. "It's the most basic level of defence: You can't defend against it, if you don't know what's there ... And we don't want to be playing Russian roulette with our military space systems."

After an intense competition that has gone on for several years and has already resulted in the awards of millions of dollars for prototypes, the high-stakes bidding has been whittled down to two competitors: defence giants Lockheed Martin and Raytheon.

"We believe the space fence is very critical to not only our country but to the world," said Steve Bruce, a Lockheed vice-president. "We do rely on space for almost everything. And without us being able to safely maintain equipment in space it would affect our worldwide economy pretty significantly ... not to mention national security."

Mike Nachshen, a Raytheon spokesman, said the company "is confident of our solution and looks forward to the Air Force's decision".

The US Air Force plans to build at least one radar system, to be located on the Marshall Islands in the Pacific Ocean. (A possible second system could be located in Western Australia).

Now, the Defence Department tracks about 20,000 items with other sensors and radars. But the new fence would be able to track as much as 10 times that debris. It would also be able to identify smaller items, some the size of a golf ball.

Armed with the catalogue of items, analysts forecast possible collisions, like the ones dramatised in the film Gravity. (But the objects are moving so fast that scientists say there's no way astronauts could see what's coming.) And for the approximately 1100 working satellites - and also the international space station and the Hubble telescope - to get out of the way, the warnings often have to come well in advance.

In 2012, the United States issued more than 10,000 warnings of close calls, to US and international satellite owners, helping with 75 "avoidance manoeuvres" - satellites expending precious energy to move out of the way.

"It's all predictive," said Brian Weeden, a former air force officer and a technical adviser for the Secure World Foundation. "They're predicting where things are going to be three, four days or a week into the future and looking for close approaches."

But there is so much debris whizzing through space that some collisions are inevitable.

Knowing that the international space station is going to be pelted by all sorts of garbage - nuts, bolts, even flecks of paint can cause damage - it was designed to be the "most heavily shielded spacecraft ever," NASA says, that "can survive impact with smaller piece of debris". NASA outfits its astronauts with spacesuits that are lined with a layer of material used in bulletproof vests to help protect them from flying projectiles.

The space shuttle was also designed to withstand the onslaught, and often returned to Earth with tiny pocks and even cracks in the windows, which were layered three deep for protection.

Space wasn't always so polluted. But since Sputnik was launched in 1957, the debris has grown. Spent rocket boosters, laden with fuel, have exploded. Defunct satellites left in orbit have decomposed. And the junk begets more junk, as crashes create more debris, which can then cause even more collisions.

Much of the debris is attributed to two events, which added thousands of new pieces of garbage to space, severely exacerbating the problem but also raising awareness about the growing clutter.

In 2007, China blew up one of its dead weather satellites, and then two years later, an active US communication satellite crashed into a defunct Russian satellite.

One answer is to find ways to limit the amount of potential debris that enters space.

Some rocket boosters are now being designed to fall back to Earth more quickly, so that within 25 years, they hit the atmosphere and disintegrate. The boosters, which carry excess fuel, now are built with vents, allowing the fuel to escape, which can help prevent the boosters from exploding.

Even satellite cameras are now designed in a way to prevent space pollution.

"In the old days you'd blow the lens cap away, and some debris would always result from that," said Dave Baiocchi, a senior engineer at the Rand Corp. "Now they tether the lens cap."

The ultimate answer, of course, is to clean things up. But that's far more difficult than it might seem, said Johnson-Freese, the Naval War College professor.

It's incredibly expensive, for one thing, and cleaning up space isn't exactly the kind of domestic program that members of Congress can rally behind. Then there are the international tensions. Countries continue to own the junk they've launched into space, and China, for example, wouldn't be thrilled if the United States decided to take down one of its satellites.

"There are legal issues," Johnson-Freese said. "There are no salvage laws in space. Even if we had the political will to [salvage junk], which I don't think we do, we couldn't bring down the big pieces because we don't own them."

Swiss scientists are working on what's been called a "janitor satellite," a sort of orbital garbage truck called CleanSpace One that would remove junk.

There's been some discussion of "putting a surcharge on every launch" that would go into a global fund and then help pay for clean-up, Baiocchi said. But it's still in the early stages.

"This is," he said, "uncharted territory."

Even though the movie Gravity raised awareness of the space debris problem, Johnson-Freese doesn't think there will be real action on removing the rubbish until there's a crash that "affects people, and the general public says, 'Why isn't my cellphone working?'"